Dynamical spin-reorientation transition in Fe/2ML Ni/W(110) ultrathin films

The spin-reorientation transition occurring in perpendicularly-magnetized films as a function of film thickness is well known. Long-range dipole interactions act to form antiparallel domains in the perpendicular phase that precedes the reorientation to in-plane magnetization via a canted phase. In equilibrium, the domains walls move freely to minimize the global energy and form a uniform striped pattern. This spatial averaging causes the reorientation to occur at a non-integer number of monolayers (ML). When the domain walls are pinned in thinner films, the system minimizes energy locally, and a metastable reorientation transition occurs on isolated islands that are 1 ML thicker than the surrounding film. These two versions of the same transition produce two separate peaks in the susceptibility χ_⊥ (in a perpendicular field), if it is measured as the film is grown. We report here observation of a dynamical version of the reorientation transition as the domain walls depin and the system moves from locally to globally determined energetics. The measured susceptibility χ₀₀₁ (in an in-plane field) exhibits a divergence. These observations provide insight to the role of dipole interactions in the transitions from the perpendicular phase to the paramagnetic and canted phases.